Xanthophyll (e.g., astaxanthin, canthaxanthin, zeaxanthin, adonirubin, adonixanthin, cryptoxanthin, and the like), which is a carotenoid, has been used for various purposes. For example, astaxanthin, which is a carotenoid imparting a red color, is known to have a potent antioxidative effect. Thus, it is used as a pigment in food, a cosmetic, a health food product, a pharmaceutical, and the like. Some astaxanthins are chemically synthesized. Astaxanthins are also naturally occurring. The naturally occurring astaxanthins are extracted from, for example, Eucarida such as euphausiids and Pandalus borealis, from Phaffia yeast, and from algae. However, astaxanthin is not produced efficiently from Eucarida such as euphausiids or yeast because of their low astaxanthin content.
On the other hand, algae, e.g., Haematococcus, which are encysted as a result of a change in the external environment accumulate astaxanthin in the algal cells. Thus, production of astaxanthin from algae has been investigated.
For example, J. Fabregas et al., J. Biotech. Vol. 89, p 66, (2001) describes a method for producing astaxanthin by cultivating Haematococcus in two steps. According to this method, in the first step, vegetative cells of Haematococcus are obtained while exchanging 10 to 40% of a culture medium every day (i.e., fed-batch culture). In the second step, a batch culture is performed for an additional 15 days under irradiation with light to induce the vegetative cells to become dormant (i.e., undergo encystment) and accumulate astaxanthin within the cells.
R. T. Lorenz et al., TIBTECH, vol. 18 (April), p 160-167, (2000) describes a two-step culture method of Haematococcus for the purpose of producing astaxanthin commercially. According this method, in the first step Haematococcus is cultivated within a sealed bioreactor under light irradiation to obtain vegetative cells. In the second step, the vegetative cells are transferred to an outdoor culture pool containing a medium in which nitrogen and phosphorus are deficient, and then the vegetative cells are induced to become dormant (i.e., undergo encystment) and to accumulate astaxanthin within the cells, by cultivating the cells while increasing a culture temperature under the irradiation with light having the elevated intensity, or by cultivating the cells in the medium of the outdoor culture pool to which sodium chloride is added.
Japanese Laid-Open Patent Publication No. 2000-60532 describes a method in which, in the first step, Haematococcus is cultured within a sealed bioreactor under irradiation with light to obtain vegetative cells, and then, in the second step, the cells are shifted to the resting state in an outdoor culture pool to induce production and accumulation of astaxanthin, and the Haematococcus is collected before the growth of predaceous or parasitic organisms on Haematococcus. 
However, with respect to the two-step methods described above, in the second step where cultivation is performed in an open or outdoor culture pool such as a raceway-type open-air culture tank, it is highly likely that various bacteria will grow in the medium. For this reason, it is necessary for the duration of the encystment period to be short, and the astaxanthin content of the encysted cells consequently is low. In order to increase the production efficiency of astaxanthin, it is necessary to prepare a large amount of vegetative cells.
Japanese Laid-Open Patent Publication No. 2004-129504 describes a method for producing astaxanthin by cultivating Haematococcus in the dark or without irradiation with light and under an aerobic condition, but this method has a problem of low astaxanthin productivity.
In view of the foregoing, there has been a demand for a method for producing astaxanthin efficiently from algae.